Imaging devices, such as an x-ray imager, have been used for diagnostic and treatment purposes. One type of x-ray imager is a diagnostic imager configured to operate with a diagnostic radiation source. Another type of x-ray imager is a high Detective Quantum Efficiency (DQE) detector that is configured for use with a treatment radiation source. An x-ray imager may also be configured for use with both diagnostic radiation beam and treatment radiation beam.
Creating a high DQE electronic portal imaging device (EPID) presents a significant technical challenge. One approach uses thick pixilated scintillator arrays that are coupled to a matrix of photodiodes. Incoming x-ray photons deposit energy into the scintillator which then produces optical photons via luminescence. These optical photons, which originate with random polarizations and direction vectors after the luminescence events, are transported throughout the scintillator where they can be reflected, refracted and scattered out of borders. Eventually, many photons will cross the boundary of the scintillator, and will reach the EPID's photodiodes. The photodiodes convert the photons into electrical current for readout and digitization. Despite the promise of the technology, performance of current EPIDs may be inadequate.
Current EPIDs employed in the field of radiotherapy utilize standard indirect flat panel design that includes a thin gadolinium oxysulfide (GOS) scintillator. The thickness of GOS scintillator is typically kept small to preserve the spatial resolution. The small thickness of the scintillator and high energies of megavoltage photon beams, used in radiotherapy field, yield low X-ray absorption within a scintillator. Low X-ray absorption limits the number of optical light produced in the scintillator and measured subsequently by a photo-diode matrix. Although a spatial resolution of the resulting digital image stays high, the signal to noise ratio (SNR) is degraded due to poor absorption characteristics. Current EPIDs suffer from low DQE (e.g., ˜1.8%) for MV imaging. Imaging tasks with such low quantum efficiency detectors are not very practical due to low contrast, especially when imaging soft tissue. Creation of an imaging device with a high DQE in the megavoltage range remains an important problem in the radiotherapy field.
One possible approach to increasing quantum efficiency (QE) is to try to make several identical detection layers for the EPID, and then stack them together. One example of such implementation is to have several identical detection layers folded under each other with the aim to catch unabsorbed X-rays in superior detection layers. While such configuration can increase DQE, manufacturing expense may make such a solution impractical. For example, if the EPID has four identical detection layers, the total DQE of the EPID can increase up to 4 times compared to a single layer EPID. The production cost may also increase up to four times, making higher efficiency benefits less attractive.